In retreat, left top herb (4)
I believe the answer is:
dill
'herb' is the definition.
(I know that dill is a type of herb)
'in retreat left top' is the wordplay.
'in retreat' is a reversal indicator.
'left' becomes 'l' (common abbreviation).
'top' becomes 'lid' (lid is a kind of top).
'l'+'lid'='llid'
'llid' in reverse letter order is 'DILL'.
(Other definitions for dill that I've seen before include "Herb of parsley family" , "Herb commonly used with gravlax" , "Herb used in cooking" , "medicinal plant" , "Medicinal herb" .)